The production of hot melt liquid adhesive foams is disclosed in Scholl et al U.S. Pat. Nos. 4,059,466 and 4,059,714, both issued on Nov. 22, 1977, and assigned to the same assignee as this application. Such foamed liquid adhesives present several advantages over non-foam adhesives. For example, after deposition a foamed liquid adhesive displays a longer "open" time period during which it remains sufficiently fluid to be receptive for adhering a substrate to it. Further, a foamed liquid adhesive displays better adhesive strength per unit weight, and thus reduces the quantity of adhesive required for a given bond.
Apparatus for producing such foams is known in the art. The Scholl et al patents cited above disclose gear pumps for this purpose. In such pumps, the liquid flow is "segmented," as discrete volumes, in the intertooth spaces of the gears. The foam-forming gas (usually air or an inert gas such as carbon dioxide, nitrogen or the like) is dispersed and apparently dissolved in the molten hot melt adhesive by the action of the meshing gear teeth increasing pressure on the segmented portions of the fluids. It will be appreciated that such gear pumps (and associated driving motors) are relatively bulky and require precision moving parts. For mix-in-gun use, the pump must be positioned at the gun or pont of dispensing.